The present invention relates to the production of aluminum by electrolytic reduction of alumina dissolved in a fused fluoride salt-bath mainly composed of cryolite. More particularly, the present invention relates to a process for eliminating the anode effect by introducing a gas below the lower surface of a carbon anode and to a process for decreasing an inter-electrode voltage. In addition, the present invention relates to an apparatus for carrying out the process.
Terms used in the electrolytic production of aluminum and the present specification are first explained.
The specific electric power consumption (P) refers to the electric power consumed for the electrolytic production of a unit amount of aluminum.
The current efficiency (.eta.) is expressed in terms of: ##EQU1##
The specific electric power consumption (P) is expressed in terms of: ##EQU2##
wherein I is the intensity direct current (kA), t is time (Hrs) for passing the current, V is a cell voltage (V) and k is the electrochemical constant (kg/KAH).
The cell voltage (V) refers to a voltage applied to each electrolytic cell.
The self baking type carbon anode refers to an anode, wherein an unbaked carbonaceous paste is fed on the lower baked carbonaceous material and is baked or solidified during its downward displacement due to the heat generated by the ohmic resistance of the carbon anode and also due to the conduction of heat from an electrolyte having a high temperature, with the result that the unbaked carbonaceous paste is provided with the function of an anode.
The inter-electrode voltage refers to the voltage applied between the anode lower surface of the anode and the cathode surface which is the upper surface of the molten aluminum in the electrolytic cell.
The inter-electrode distance refers to the distance between the anode surface and the cathode surface of an electrolytic cell.
The inter-electrode voltage includes an alumina-decomposition voltage, an overvoltage and a voltage drop by the inter-electrode ohmic resistance.
The inter-electrode ohmic resistance and the voltage drop caused thereby involve two kinds. The first is the resistance of the electrolyte, hereinafter referred to as the electrolyte ohmic resistance, and the voltage drop caused by this ohmic resistance. Ohmic resistance of the electrolyte is inversely proportional to the specific electric conductivity of the electrolyte and is proportional to the inter-electrode distance. The second is a so-called gas-film resistance and the voltage drop due to this resistance. The gas-film resistance is closely related to the wettability of the carbon anode by the electrolyte, which is known.
The critical alumina concentration refers to the alumina concentration, which is gradually decreased, in accordance with the development of the electrolytic reaction, due to the decomposition of alumina dissolved in the fused fluoride salt, at which concentration the cell voltage is abruptly increased from the normal operation voltage, for example approximately 4 volts, to a voltage a few times higher, for example from 20 to 40 volts. In other words, the critical alumina concentration indicates the alumina concentration in the electrolyte which causes a critical increase of the gas-film resistance and the anode effect.
In the operation of an electrolytic cell of aluminum, it is very important to reduce the specific electric power consumption (P). Since, as described above, the specific electric power consumption (P) is proportional to the cell voltage (V) and is inversely proportional to the current efficiency (.eta.), the electrolytic cell is desirably operated under the low cell voltage (V) and high current efficiency (.eta.). In the routine operation of the electrolytic cell, in which the structure of the conductors of the cell and the composition range of the electrolyte cannot be optionally altered, the cell voltage (V) is controlled essentially by the inter-electrode voltage. That is, at a low inter-electrode voltage, the cell voltage becomes also low. The inter-electrode voltage is adjusted mainly by vertically displacing the carbon anode and thus changing the inter-electrode distance.
Incidentally, experience and theories in the electrolytic production of aluminum have indicated that: the inter-electrode distance influences the current efficiency; and, the current efficiency is decreased with the decrease in the inter-electrode distance. Accordingly, the specific electric power consumption (P) can be improved by an increase in the inter-electrode distance at a given inter-electrode voltage. The present invention is the result of research in an attempt to achieve an increase in the inter-electrode distance at a given inter-electrode voltage.